Aptamer Therapy for Macular Degeneration
Aptamers: Aptamers are Oligonucleotide or peptide sequences that bind to specific target molecules. Aptamers can be broadly classified into: 1) Nucleic acid aptamers: DNA, RNA, XNA or nucleic acid analouge aptamers. 2) Peptide aptamers. Aptamers form stable three dimensional structures that are capable of binding with high affinity and specificity to a variety of molecular targets. The interactions between proteins and nucleotides occurs naturally during replication, transcription, translation and RNA interference.This forms the theoretical basis for aptamers and aptamer therapy 1. Discovery Process: Aptamers are discovered using a process known as Systematic evolution of ligands by exponential enrichment (SELEX) 1. The process depends upon a large library of single stranded oligonucleotide templates optained via chemical synthesis and the DNA contains fixed 5' and 3' terminal sequences to enable manipulation of the internal 30-40 nucleotides 1. The library of nucleotides is then incubated with target protein and bound members of the library are partitioned and amplified to enrich the pool of bound aptamers. This selection process is repeated iteratively over several cycles to recover aptamers that bind specifically and with high affinity towards the target. Generally all the SELEX process takes 7-15 rounds of selection 1 (Fig 1). Then stabilizing modifications are incorporated into the selected aptamers to confer resistance against nucleases in the cells. Macugen treatment for Age Related Macular Degeneration (AMD) FDA approved pegatinib sodium (Macugen) as an anti Vascular endotheilial growth factor (VEGF) RNA aptamer treatment for AMD .VEGF-A as a target for treament for AMD was based on its established role in regulation of angiogenesis in cancers, ocular neocascular diseases and rheumataoid arthritis 2. VEGF was found to be elevated in several occular neovascular syndromes and animal studies suggested that elevation of VEGF causes neovasularization and vessel leakage. Scientists at NeXstar pharmaceutials carried out three seperate iterations of SELEX to enrich for aptamers targeting VEGF and their earliest work showed that the isolated aptamers were effective in blocking VEGF actions invitro. After several modifications such as NH2 substitited nucleotides and F-substitution to increase stability o fthe aptamers, one was selected for development as pegatinib 2. Mechanism: Known mechanisms of actions of pegatinib are 2: 1) Inhibition of VEGF binding to the target receptor on cells, pegatinib was found to bind to VEGF in the heparin binding domain at cysteine -137. (Fig2) 2) Prevents VEGF mediated phosporylation of VEGFR2 and Phospholipase C-gamma and VEGF induced calcium mobilization. Injection of pegatinib inhibited the pathological form of neovasularization but not physiological indicating the treatment is effective in pathological conditions and probably with lower adverse effects in normal conditions. Clincal Trials: Pegaptanib was indicated in the treatment of wet AMD as it targets choroidal neovascularization, thehallmark of wet AMD. The safety and efficacy of pegaptanib in the treatment of choroidal neovascularization secondary to AMD were tested in two concurrent, identically designed, prospective, randomized, double-masked, multicentre, dose-ranging pivotal trials. During the trial Pegaptanib sodium 0.3, 1 or 3 mg by intravitreous injection or sham injection was administered every 6 weeks for 48 weeks, a total of nine treatments77. the findings of the VISION trials demonstarted that pegaptanib treatment was able to reduce vision loss by approximately 50% in the first year and stabilized vision in the second year.After one year 55% of patients receiving sham injections, 70% of patients receiving 0.3 mg of pegaptanib, 71% receiving 1 mg and 65% receiving 3 mg lost <15 letters of visual acuity or approximately three lines on the study eye chart between baseline and week 54 (primary efficacy endpoint) (Table 1) indicating that the treatment was effective and dose dependent. All during the trial there were no Adverse reactions but had transient minor side effects. Future Developments: While most of the treatments for other opthalmic conditions has been via topical application of eye drops, AMD is a posterior condition, topical delivery is not feasible. The current treatment using pegatinib is via intravenous injection it needs a higher dose to deliver maximum effective dose at site of interest. Several alternative strategies are under development including implants to deliver small amounts and lipophillic intraocular delivery systems etc. Other Indications: Other aptamers are under development for treament of cancers, anticoagulation etc, for example AS1411 is in Phase -I trials in patients with advanced cancers and ARC183 an antithrombin aptamer is also in Phase-I trials 2 Refences: 1 Bouchard, P. R., et al. (2010). "Discovery and development of therapeutic aptamers." Annu Rev Pharmacol Toxicol 50: 237-257. 2 Ng, E. W., et al. (2006). "Pegaptanib, a targeted anti-VEGF aptamer for ocular vascular disease." Nat Rev Drug Discov 5(2): 123-132.